I went ahead and acquired a Marklin BR 44 locomotive. In saummary, it runs very well, but not without some work first. The risk is that Marklin does not import this locomotive into the US and so there is essentially no warranty, so be prepared to make your own repairs.
I first ran the locomotive on air, using the water filler tube as the air port. With the throttle closed, the boiler held air perfectly; no leaks. Opening the throttle and operating the servos manually resulted in smooth forward motion, and slightly rougher, but still reasonably smooth backward motion. I then tried running the locomotive while holding a driver wheel. The torque delivery was very uneven, and when the movement was stopped, I could hear air escaping somewhere, meaning it was not supplying any force on the pistons. I also noticed that the right hand cylinder was moving under load, due to loose mounting screws. To tighten the screws, I had to dissemble the locomotive, which I did. A parts diagram from the Marklin web site was very hgelpful for this. Especially for identifying the screws hidden under plastic hatches in the running boards. With the locomotive apart, I tried moving the pistons with the valves closed and found zero leakage. Just as a precaution, I tightened down the spring holding the reversing valve together to the point where it was as tight as I rhought it could be without overloading the serevo. I then applied air to the whole cylinder/reversing valve assembly and found zero leakage. After reassembling the locomotive (using Loctite) I again tested it on air. This time, the torque was much more even, and there was no leaking air sound when I stopped the drivers. At this point I put in batteries and tested the servos. Nothing happened, and i traced the problem to a faulty transmitter. Using another (temporary) radio/receiver, I was finally ready to steam the locvomotive.
Marklin does not supply a butane filler adapter, and recopmmends purchasing a European camping stove, taking it apart, and using certain components as a filler assembly. Luckily, I was able to fill the butane tank with an Accucraft adapter. I ran the locopmotive in 20 dergree weather. The burner lighted easily, but with an external flame. The internal lighter, although it sparked, did not light the flame (maybe I have to reposition it). The water boiled in less than five minutes, and the locomotive started on its own without pushing. I turned down the flame to where I could barely hear it with the locomotive stopped, and not at all when running. Even so, the pressure was fully up to 60 pounds, and the saftey valve was blowing he whole time. The butane tank is under the boiler and it stayed warm enough to supply butane during the entire run; I used a 70% butane/30% propane mix. Speed and direction control were excellent, the runnining was very smooth both in forward and reverse, and there were no derailments even though my ground level track was quite uneven. For a train, I used a set of five German ore hoppers (the "Long Henry" set Marklin made for this locomotive). They were pulled without any effort at all, and i will get another set of cars to make a credible looking train; but the locomotive can probably pulll at leaat 40 of these cars. By the way, theee cars are great; they cone with loads of real iron ore, are lightly weathered, and it is very easy to move the couplers in so the buffers are almost touching.
Overal, I am very pleased with the locomotive and plan to suprerdetail it by making covers for the cylinders (they come as "naked" Regener assemblies) and maybe open up some of the holes in the frame. One final note; the leaky brass boiler thread is actually about a Regner boiler; and it appears the Marklin/Regner boiler is also brass. Regner recommends that distilled water, not deionized water be used, and evan a small amount of tap water be added to the boiler water. This helps to prevent dissolution of the zinc out of the brass. How this will hold up in the long term I cannot say.